Induction heating apparatus such as induction furnaces for heating or melting metals operate on the principle of inducing eddy currents in a workpiece (sometimes referred to as the load) to be heated. The eddy currents cause the load to act as its own heat source by the P=I.sup.2 R heating principle. The eddy currents are induced in the load by passing alternating current through a generally helical induction coil disposed near or around the load.
Induction furnaces in common use today include induction coils of copper tubing adapted to allow a liquid coolant to flow therethrough. The copper tubing conducts the alternating current which produces the electromagnetic field inside the furnace to create the eddy currents in the load. Running water or other liquid coolant flows through the copper tubing of the coil to remove the heat conducted through the refractory material and the heat generated by the coil current.
The efficiency of an induction furnace depends, in part, on the amount of energy (in the form of electromagnetic energy) which couples from the induction coil to the load and is converted into heat energy in the load. One overall goal in designing such furnaces is to maximize this efficiency. The efficiency is a function of many different design parameters. One parameter which affects the efficiency is the tubing used to fabricate the induction coil. Different tubing shapes, sizes and dimensions, when wound into a helical induction coil, will produce different electromagnetic flux patterns. Different patterns will cause more or less of the electromagnetic energy to couple into the load, thereby resulting in greater or lesser furnace efficiency.
In the prior art, the induction coil tubing typically has a rectangular cross-sectional profile with a rectangular or round opening for cooling fluid to flow therethrough. The outer side walls of the rectangular tubing are typically straight, although sometimes the outer corners may be slightly rounded. Another well-known form of tubing has a circular cross-sectional profile with a circular opening therethrough. Oval-shaped tubing with an oval-shaped opening therethrough is also well-known.
One prior art attempt to increase the efficiency of an induction coil by changing the geometry of the tubing involved displacing the opening of the tubing away from the center axis of the tubing. In other words, instead of the geometric center of the opening being centered on the center axis of the tubing, the geometric center of the opening was spaced apart from the center axis. This displacement resulted in a reduction of losses due to a decreased amount of energy lost in the tubing.
In spite of extensive research and exhaustive attempts to further improve the efficiency of induction melting furnaces, there is still a need for further improvements in efficiency of an induction coil so as to maximize the proportion of energy supplied to the induction coil which couples to the load and heats it through induced eddy currents. Specifically, there is a need for further improvements in the shape of the tubing used in the induction coil which will lead to higher efficiencies. The present invention fills that need.